AXL is a membrane bound receptor tyrosine kinase and is the founding member of the TAM (Tyro3, AXL, Mer) family that is characterized both by their two immunoglobulin-like domains and the dual fibronectin repeats found in their extracellular domain and by their related tyrosine kinase domains found in their cytoplasmic domain. (Linger, R. M. et al., TAM receptor tyrosine kinases: biologic functions, signaling, and potential therapeutic targeting in human cancer. Advances in cancer research 2008, 100, 35-83.) Extracellular signaling mediated by TAM receptor tyrosine kinases has been implicated in a variety of normal cell functions such as cell survival, migration and adhesion. There are two known ligands for the TAM family, GAS6 (growth arrest specific-6) and protein S. Gas 6 binding to AXL results in receptor dimerization and AXL auto-phosphorylation. (Stitt, T. N. et al., The anticoagulation factor protein S and its relative, Gas6, are ligands for the Tyro 3/Axl family of receptor tyrosine kinases. Cell 1995, 80 (4), 661-70.) Also, the extracellular domain can be shed, though the function of the soluble TAM extracellular domains is unknown. Mouse knockouts of TAM family members have confirmed that they play roles in innate immunity, inflammation, phagocytosis and cell differentiation. (Cohen, P. L. et al., Delayed apoptotic cell clearance and lupus-like autoimmunity in mice lacking the c-mer membrane tyrosine kinase. The Journal of experimental medicine 2002, 196 (1), 135-40; and Lemke, G. et al., Macrophage regulation by Tyro 3 family receptors. Current opinion in immunology 2003, 15 (1), 31-6.) AXL is ubiquitously expressed, having been detected in a wide variety of organs and cells, including cell lines of epithelia, mesenchymal and hematopoietic origins, as well as non-transformed cells. (O'Bryan, J. P. et al., Axl, a transforming gene isolated from primary human myeloid leukemia cells, encodes a novel receptor tyrosine kinase. Molecular and cellular biology 1991, 11 (10), 5016-31.)
There is extensive literature implicating the dysregulation of AXL and other TAM family members in the development of cancer. AXL (short for “anexelekto” which means uncontrolled) was first discovered as the transforming oncogene in two chronic myelogenous leukemia (CML) patients. (Liu, E. et al., Transforming genes in chronic myelogenous leukemia. Proceedings of the National Academy of Sciences of the United States of America 1988, 85 (6), 1952-6.) AXL has been implicated in a wide range of cellular functions potentially important in cancer development, ranging from cell survival, angiogenesis, metastasis and immunity. (Li, Y. et al., Axl as a potential therapeutic target in cancer: role of Axl in tumor growth, metastasis and angiogenesis. Oncogene 2009, 28 (39), 3442-55; and Linger, R. M. et al., Taking aim at Mer and Axl receptor tyrosine kinases as novel therapeutic targets in solid tumors. Expert opinion on therapeutic targets 2010, 14 (10), 1073-90.)
Overexpression of AXL is associated with poor prognosis and increased invasiveness of human cancers and has been reported in breast (Berclaz, G. et al., Estrogen dependent expression of the receptor tyrosine kinase axl in normal and malignant human breast. Annals of oncology: official journal of the European Society for Medical Oncology/ESMO 2001, 12 (6), 819-24; and Zhang, Y. X. et al., AXL is a potential target for therapeutic intervention in breast cancer progression. Cancer research 2008, 68 (6), 1905-15), colon (Craven, R. J. et al., Receptor tyrosine kinases expressed in metastatic colon cancer. International journal of cancer. Journal international du cancer 1995, 60 (6), 791-7), esophageal (Nemoto, T. et al, Overexpression of protein tyrosine kinases in human esophageal cancer. Pathobiology: journal of immunopathology, molecular and cellular biology 1997, 65 (4), 195-203), thyroid (Ito, T. et al, Expression of the Axl receptor tyrosine kinase in human thyroid carcinoma. Thyroid: official journal of the American Thyroid Association 1999, 9 (6), 563-7), ovarian (Sun, W. et al., Coexpression of Gas6/Axl in human ovarian cancers. Oncology 2004, 66 (6), 450-7), gastric (Wu, C. W. et al., Clinical significance of AXL kinase family in gastric cancer. Anticancer research 2002, 22 (2B), 1071-8), renal (Chung, B. I. et al., Expression of the proto-oncogene Axl in renal cell carcinoma. DNA and cell biology 2003, 22 (8), 533-40), glioma (Hutterer, M. et al., Axl and growth arrest-specific gene 6 are frequently overexpressed in human gliomas and predict poor prognosis in patients with glioblastoma multiforme. Clinical cancer research: an official journal of the American Association for Cancer Research 2008, 14 (1), 130-8) and lung cancers (Shieh, Y. S. et al., Expression of axl in lung adenocarcinoma and correlation with tumor progression. Neoplasia 2005, 7 (12), 1058-64.)
AXL over-expression is closely associated with epithelial-mesenchymal transition (EMT) which is the process by which epithelial cells become motile and invasive and that often is a key characteristic of late stage metastatic tumors. (Byers, L. A. et al., An epithelial-mesenchymal transition gene signature predicts resistance to EGFR and PI3K inhibitors and identifies AXL as a therapeutic target for overcoming EGFR inhibitor resistance. Clinical cancer research: an official journal of the American Association for Cancer Research 2013, 19 (1), 279-90.) EMT also confers resistance to oncogene mediated senescence which is a key step in tumor development. (Smit, M. A. et al., Epithelial-mesenchymal transition and senescence: two cancer-related processes are crossing paths. Aging 2010, 2 (10), 735-41; and Thiery, J. P. et al., Epithelial-mesenchymal transitions in development and disease. Cell 2009, 139 (5), 871-90. Furthermore, AXL has also been implicated in playing a role in resistance to both chemotherapy and targeted therapies. (Neel, D. S. et al, Secrets of drug resistance in NSCLC exposed by new molecular definition of EMT. Clinical cancer research: an official journal of the American Association for Cancer Research 2013, 19 (1), 3-5; and Zhang, Z. et al., Activation of the AXL kinase causes resistance to EGFR-targeted therapy in lung cancer. Nature genetics 2012, 44 (8), 852-60.)
AXL potentially plays an important role in many diverse key tumorigenic mechanisms but as a result it has not been clear how to genetically select patients or when to treat during the development of cancer with an AXL inhibitor, for instance when during their tumor development is AXL function is actually required, and in which patients should an AXL-based therapeutic be used. These two fundamental unanswered questions have held up the development of AXL inhibitors for the treatment of cancer. Indeed finding a key AXL dependent functions and relevant preclinical models where AXL drives tumor cell survival has been a major challenge.